An extracorporeal blood circuit often experiences variations in the flow rate of blood, for example, when the amount of blood drained from the patient decreases or when the amount of blood fed to the patient must be increased. An extra amount of blood should be stored in the circuit to accommodate such variations in blood flow rate. To this end, a blood reservoir is inserted in the circuit for temporarily storing venous blood drained from the patient. This type of reservoir is often called a venous reservoir.
One typical venous reservoir includes a pair of plastic sheets sealed along their four sides for defining a closed space therein. This closed type of venous reservoir has no risks of contact with air, entry of foreign matters, or pumping-in of air even when the amount of blood drained decreases.
The venous reservoir should preferably have the following functions.
(a) Blood storage function: The reservoir should have a sufficient volume or space to accommodate a necessary volume of blood.
(b) Debubbling function: Air bubbles can be introduced into a drainage tube when a blood drainage catheter is unsteadily dwelled in the vein or when the catheter is withdrawn from the vein. It is required to remove air from the drained blood in the reservoir.
(c) No stagnation: The reservoir should be free of local stagnation of blood.
Requirement (b), debubbling or air removal is a critical factor. If the reservoir does not function well in air removal, there is a possibility that air be introduced into the patient, causing thrombi in blood capillaries of various organs of the patient, particularly of the brain, eventually inviting cerebropathy after the operation and leaving a life threatening crisis.
In general, debubbling ability is improved as the blood storage volume of the reservoir increases. However, it is not recommendable to increase the blood storage volume. An increased blood storage volume increases the priming quantity of an overall extracorporeal circuit and requires a correspondingly increased amount of blood transfused, which increases the possible occurrence of hepatitis after the operation. An increased blood storage volume is also undesirable from the standpoint of blood saving.
For this reason, efforts have been made to develop a blood reservoir capable of efficient removal of air bubbles without increasing the blood storage volume.
One such attempt is the venous reservoir disclosed in Japanese Patent Application Kokai No. 67962/1984. Referring to FIG. 15, there is illustrated a venous reservoir at 100 as comprising a flexible container casing 101 defining a blood storage space 102 therein. The casing 101 is formed by sealing a pair of plastic sheets along their four sides. A blood inlet port in the form of a tube 105 is attached to the lower portion of the casing 101 such that the distal end 105a of the tube protrudes into the space 102. The distal portion 105a of the inlet tube 105 is provided with a plurality of apertures 106 so that drainage blood may be introduced into the space 102 through the inlet tube 105 in a dispersed manner. Then blood moderately flows into the space 102 from the inlet tube 105, contributing to maintenance of an environment where air bubbles will rise vertically upward through the blood under the impetus of buoyancy of bubbles themselves.
A blood outlet port in the form of a tube 107 is attached to the lower portion of the casing 101 at a predetermined spacing from the inlet port 105 and in parallel with the inlet port 105. A vent port 108 is disposed at the upper portion of the casing 101 in communication with the blood storage space 102. The casing 101 includes an extra sealed portion 101c defining a slant upper wall for the space 102 so that rising air bubbles may move along the slant upper wall 101c toward the vent port 108. Air bubbles escape outside the casing 101 through the vent port 108.
The venous reservoir 100 of the above-described construction has a likelihood of occurrence of a blood short-circuit that part of blood entering the space 102 from the apertures 106 in the inlet tube 105 directly flows toward the outlet port 107. Then some blood can be discharged or fed back to the extracorporeal circuit through the outlet port 107 without being fully debubbled. As a whole, this reservoir shows a less sufficient debubbling function.
The venous reservoir 100 of the prior art has an additional drawback that blood tends to locally stagnate in corners T near the bottom and the top of the space 102 surrounding the inlet tube 105 and the vent port 108, respectively.
There is a need for an improved blood reservoir which has overcome the above-mentioned problems.